Looked at the pagehttps://trac.ffmpeg.org/wiki/How to speed up / slow down a video
I'm able to slow down a video-audio by the half of the original speed using the command
ffmpeg -i input.mkv -filter_complex "[0:v]setpts=2.0*PTS[v];[0:a]atempo=0.5[a]" -map "[v]" -map "[a]" output.mkv

The problem is that the half of the original speed is too much, I would need a bit faster.
So I tried the command:
ffmpeg -i input.mkv -filter_complex "[0:v]setpts=1.5*PTS[v];[0:a]atempo=0.75[a]" -map "[v]" -map "[a]" output.mkv
but the problem here is that the resulting video is out of sync.